UV Unwrapping a Modified Model
This kind of distortion often happens when you modify the mesh that has been unwrapped. You can try to unwrap once again after modification, or pin the vertices of islands you want to stay in the same position and then unwrap. In UV/Image Editor window select all the vertices except the distorted one and press P.


Now select the whole mesh (A) in a 3D View window and unwrap it. Being in UV/Image Editor window unpin all using ALT+P shortcut. The modified vertices should come back to their places.